Among the wide variation in techniques available to process semiconductor substrates are plasma deposition processes. These processes are similar to conventional chemical-vapor-deposition (“CVD”) techniques in which reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to process the substrate, either by depositing a desired film or by etching the film depending on the chemistry. Plasma CVD techniques such as plasma-enhanced CVD (“PECVD”) increase the reactivity near the substrate by promoting excitation and/or dissociation of reactant gases through the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface. Because o the increase in reactivity, the energy needed for a chemical reaction to take place is reduced, allowing the temperature for such processes to be lower than for thermal CVD processes.
There are different known methods of coupling the energy from an RF source to the plasma. In one class of methods, the coupling is “capacitive.” Electrodes are provided within a process chamber, with the RF source applying a voltage between the electrodes. The plasma is formed between the electrodes and forms sheaths of relatively low electron density near solid surfaces. The RF voltage appears primarily across these sheaths as if they were the dielectric region of a capacitor.
One consequence of such capacitive coupling is the generation of an impedance R−j/ωC for resistance R and capacitance at frequency ω, where j denotes √{square root over (−1)}. Some compensation for this impedance may be provided with a fixed matching network, but the fixed nature of the matching network provides only a uniform compensation. The actual impedance associated with the capacitive coupling may be highly variable, depending on the specific processing characteristics that are used. There is accordingly a need in the art for a matching network capable of accommodating a variety of different processing characteristics for capacitively coupled plasmas.